Passivity and Pitting Corrosion of X80 Pipeline Steel in Carbonate/Bicarbonate Solution Studied by Electrochemical Measurements H.B. Xue and Y.F. Cheng (Submitted September 23, 2009; in revised form February 17, 2010) This work investigated the effects of chloride ions and hydrogen-charging on the passivity and pitting corrosion behavior of X80 pipeline steel in a bicarbonate-carbonate solution by electrochemical and photo- electrochemical techniques. It was found that a stable passivity can be established on the steel in the absence and presence of chloride ions. The hydrogen-charging does not alter the transpassive potential, but increases the passive current density. When chloride ions are contained in the solution, pitting corrosion will be initiated. The pitting potential is independent of the hydrogen-charging. Hydrogen-charging would enhance the anodic dissolution and electrochemical activity of the steel, but does not affect the pitting potential, which indicates that the charged hydrogen is not involved in the pitting initiation. However, hydrogen may accelerate the pit growth. Photo illumination could enhance the activity of the steel electrode, resulting in an increase of photo-induced anodic current density. Keywords bicarbonate-carbonate solution, passivity, pitting cor- rosion, X80 steel 1. Introduction It has been acknowledged (Ref 1-16) that a majority of pipeline stress corrosion cracking (SCC) occurs under a high pH condition, which is associated with a concentrated carbonate/ bicarbonate solution with a pH of about 9.5. The high pH SCC of pipelines is believed to be due to the selective dissolution and repeated rupture of passive film along grain boundaries (Ref 3, 7-9). Generally, pipeline steel can be passivated in the concentrated carbonate/bicarbonate solution (Ref 17-19). In the presence of aggressive species, i.e., Cl  ions, the passive film may be broken to initiate pitting corrosion. It has been reported (Ref 20, 21) that corrosion pits are usually the incubators of stress corrosion cracks in pipeline steel. It has been demonstrated (Ref 22-26) that the passive film formed on pipeline steel in concentrated bicarbonate/carbonate solution behave like a semiconductor, and the semiconducting properties of the film play an essential role in corrosion and SCC processes of the steel (Ref 27, 28). Furthermore, the presence of hydrogen in pipeline steel influences strongly its chemical and mechanical properties (Ref 29-32). Also, the transport of hydrogen through passive film affects many corrosion processes, such as pitting corrosion and SCC (Ref 29, 32). Recent investigations (Ref 33, 34) showed that hydrogen, as a product of corrosion reaction, has significant influence on the composition and structure of passive film, and results in an increased pitting susceptibility and a decreased corrosion resistance of the film. Photo-electrochemical technique provides a promising alternative to study the semiconducting properties of passive film, and simultaneously, to characterize in situ the effect of hydrogen in steel on its corrosion and cracking behavior (Ref 35-38). For example, Razzini et al. (Ref 35) used a photo- electrochemical microscopy (PEM) to image the distribution of hydrogen at the crack tip in a loaded X60 pipe steel specimen after hydrogen-charging. It demonstrated that a large amount of hydrogen segregates at high stress regions. Zeng et al. (Ref 39, 40) used photo-electrochemical measurements to investigate the effects of hydrogen and chloride ions on the electronic properties of passive film on X70 steel in 0.5 M NaHCO 3 solution. It found that hydrogen could promote localized corrosion. The X80 steel is a high-strength steel and is being used in pipeline operations in Arctic and sub-Arctic areas (Ref 41). Development of an advanced understanding of the corro- sion and SCC behavior of X80 steel is critical to the safe operation of pipelines. In this work, the passivity and pitting corrosion behavior of X80 steel were investigated in a carbonate/bicarbonate solution by electrochemical and photo- electrochemical measurements. The effects of chloride ions and hydrogen on the properties of passive film and pitting corrosion were determined. 2. Experimental 2.1 Electrodes and Solutions The specimens used for this work were a X80 steel, with the chemical composition (wt.%): C 0.026, Mn 1.6, Si 0.22, H.B. Xue and Y.F. Cheng, Department of Mechanical and Manufac- turing Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada. Contact e-mail: fcheng@ucalgary.ca. JMEPEG (2010) 19:1311–1317 ÓASM International DOI: 10.1007/s11665-010-9631-3 1059-9495/$19.00 Journal of Materials Engineering and Performance Volume 19(9) December 2010—1311